This invention relates in general to wireless communication systems, and more specifically to a method for reducing intermodulation distortion in a radio receiver.
A mobile radio in the field will find that the modern environment for the reception of radio signals requires very aggressive performance standards. This is because of a number of factors. The large number of devices transmitting simultaneously in the field today causes interference. The desire to keep infrastructure costs down, and thus mobile range high, requires large dynamic range radios. The growth of superstructures (such as buildings) which block and reflect signals causing fading and simulcasting radio patterns. Extending battery life of a portable, or mobile, radio can only be accomplished through keeping power consumption down.
For simplicity, a mobile radio generally consists of an antenna followed by: a step attenuator, a low noise amplifier (LNA), down conversion mixers, digital to analog conversion, channel filtering, an input to an AGC, and other digital signal processing for the purpose of demodulating and interpreting the signals.
The signal enters the radio through the antenna and the strength of this signal can vary over 120 decibels in some applications. The demodulator in the back end of the radio cannot deal with large variation in signals. In order to deal with the wide dynamic range requirements and fading signals, radios are designed with automatic gain control (AGC). The automatic gain control takes the form of a negative feedback loop wrapped from the power monitoring point in the rear of the radio to the front end gain stage of the radio. The front gain stage of the radio consists of the low noise amplifier (LNA) and step attenuator under control of the AGC controller. The automatic gain control (AGC) monitors the power of the signal received at the back end of the radio. The signal power is measured against a predefined threshold. The desire is to maintain the signal at this power level (specified by this threshold) to correctly demodulate and interpret the data. If the signal is below the threshold, the AGC will want to raise the gain of the front end of the radio (LNA). If the signal is above the threshold, the AGC will want to lower the gain of the front end of the radio. If the signal is too low, the AGC will shift all the way to maximum gain mode. If the signal is too high, the AGC will shift all the way to a minimum gain mode.
In order for mobile radios, to operate with longer battery life, it is necessary to lower the current consumption of radio stages. Traditionally the low noise amplifier (LNA) in the front of the radio is the dominate current drain stage. To significantly lower the current drain of that stage would dramatically reduce the current drain of the entire radio and substantially improve battery life. Extremely, low current design LNAs however present another problem to the radio design: non-linearity.
The linearity of the LNA is important because unwanted signals entering the antenna can mix into the passband of the radio. An unwanted signal in the passband will interfere with the demodulation process and not allow the radio to interrupt the data correctly. This type of interference is called intermodulation. A measure of the linearity of a LNA is called the third order intercept point (IP3). A low noise amplifier with a higher IP3 performance is more linear and creates less interfering component in the pass band of the radio. Traditionally, LNA designs have run at higher current levels because they wanted to maintain this high linearity performance. These products, of course, had larger batteries to compensate. In today""s radios which seek extremely low power, the LNA can develop problems with linearity. Specifically in a paging radio, or selective call receiver, or pager, the LNA design runs at five times less current than traditional LNA designs of those skilled in the art. This very low current design, although great for improving the battery life of the product, has a peaking response in the intercept point performance (IP3) over the gain range of operation of the amplifier. Over the entire gain range of operation of the amplifier (from minimum to maximum gain) the IP3 performance peaks in the middle of the operation range. During this peak, the IP3 and thus the intermodulation performance of the radio, is great. But to either side of the peak the intermodulation performance drops greatly, typically below specifications.
The peak in the IP3 performance leads to another problem in a traditional closed loop negative feedback AGC system. The peaked IP3 response will force the AGC to lock to the minimum gain condition where the intermodulation performance is the worse and the data being transmitted would be lost. For example, if the radio is placed in a strong signal environment with other channels present to create intermodulation products in the passband. The AGC system averages the power of both the wanted and the unwanted intermodulation (IM) components in the passband. In a strong signal scenario at the input to the antenna, the AGC system will want to turn the gain down. As the gain gets cut back from maximum toward minimum, the radio moves toward the peak in the LNA IP3 performance. At the peak, the IM product reduces in the passband because this is the most linear operation point of the amplifier. If the power out of the back end of the radio is still strong enough to be above the AGC threshold, the AGC system will want to continue to reduce the gain. Now the radio is moving off the peak of the LNA IP3 curve and IM product in the passband starts to increase even though the wanted signal is decreasing because the over all gain of the LNA is being lowered. Because the IM product in the passband is increasing the AGC system measures its power and continues to want to reduce the gain. (The AGC power detector measures the power in the complete passband of the radio.) The radio will continue to reduce gain until it hits its minimum value. This is a detriment to the radio because now the IM component is stronger than the wanted component in the pass band and the data to be received is lost.
Thus, what is needed is a method of reducing intermodulation distortion in a low current drain automatic gain control system, and more particularly, a method and apparatus of overcoming the problem with peaked IP3 response and AGC locking at an incorrect gain.